1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus.
2. Description of the Background Art
Optics included in an image forming apparatus has been improved in various ways in order to meet an increasing demand for higher resolutions. However, to implement different resolutions with a single sheet conveying speed, optics with an advanced function is essential, resulting an increase in cost. In light of this, some image forming apparatuses are configured to realize a high resolution by varying the sheet conveying speed. For example, the sheet conveying speed is halved to double the resolution, as proposed in the past.
As for an electrophotographic image forming apparatus, a thermal fixing device is extensively used that fixes a toner image formed on a paper sheet, OHP (OverHead Projector) film or similar sheet-like recording medium. It is a common practice with a thermal fixing device to press a press roller against a heat roller, which is heated by a heater or heat source accommodated therein. The heat roller and press roller fix a toner image on a sheet with heat and pressure while conveying the sheet. In this type of thermal fixing device, heat generated by the heater for a unit time is usually controlled to maintain the surface temperature of the heat roller at a preselected fixing temperature so as to apply adequate heat to a sheet being conveyed via a nip between the two rollers. If the heat applied to the toner image is excessive, then toner grains are melted by heat to bring about a spot-off problem or the sheet creases. If the heat is short, defective fixation occurs due to the insufficient softening of toner grains.
As stated above, if the condition of the fixing temperature is not varied when the sheet conveying speed is varied to implement a high resolution, then spot-off and creasing or defective fixation occurs due to the variation of heat applied to a sheet.
To solve the above problem, a target fixing temperature may be raised when the sheet conveying speed is high, i.e., when the resolution is low or lowered when the sheet conveying speed is low, i.e., when the resolution is high. This kind of scheme, however, brings about another problem that at the time when the sheet conveying speed is varied, fixation has already started or a sheet has already been driven out of the apparatus. The resulting excessive heat or short heat renders the resulting image defective. While sheet feed may begin after the temperature has reached a target fixing temperature, the operator of the apparatus must simply waste such a period of time, resulting in low productivity.
The problem discussed above also occurs with temperature control to be executed in the stand-by state of the apparatus. Specifically, it has been customary with an image forming apparatus operable with different sheet conveying speeds to selectively set up a particular fixing temperature and a particular sheet conveying speed for each of a low resolution and a high resolution. Generally, the low resolution is frequently used while the high resolution is assigned to special prints needing high quality. As for a stand-by state, to reduce the waiting time until the output of the first copy, a single, relatively high fixing temperature is assigned to the fixing unit in matching relation to the low resolution (high fixing temperature), which is more frequently used than the high resolution. For example, while fixing temperatures of 180xc2x0 C. (low resolution or high sheet conveying speed) and 140xc2x0 C. (high resolution or low sheet conveying speed) are available for sheet feed, only a stand-by temperature of 160xc2x0 C. is available for the stand-by state. In this case, a person desiring the low sheet conveying speed must simply wait until the fixing unit has been cooled off to the adequate fixing temperature of 140xc2x0 C. This critically lowers the productivity of the apparatus.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2-120781.
It is an object of the present invention to provide an image forming apparatus capable of varying a sheet conveying speed and yet applying adequate heat to a sheet while reducing a waiting time.
An image forming apparatus of the present invention includes a thermal fixing unit and is capable of varying a sheet conveying speed. The sheet conveying speed is varied in accordance with a resolution selected. The target fixing temperature of the fixing unit is variable in accordance with the sheet conveying speed. Further, the stand-by temperature of the fixing unit after image formation is set in accordance with the resolution selected to thereby control a temperature. This successfully reduces the operator""s waiting time and applies adequate heat to a sheet.